This invention relates to synthetic functional fluids and more particularly to synthetic functional fluids containing an ester formed from the reaction of a polyol with certain monocarboxylic acid compositions.
It has been discovered that functional fluids of this type are particularly adapted for use in automatic transmissions.
The fluid in an automatic transmission serves several functions. It serves as a power-transmission fluid in the torque converter, an hydraulic fluid in the control and servo systems, a lubricant for the bearing and gears, and a friction-controlling medium for the bands and clutches. In addition, it serves as a heat-transfer medium to carry the heat generated in the transmission to the wall of the torque converter, which is cooled by air circulation, or to a separate water-cooled heat exchanger.
Operating temperatures in many newer transmissions have increased sharply. In many cases, transmission drain intervals have been extended or eliminated entirely. These factors have led to the development of new and improved transmission fluids.
Petroleum lubricants, which have heretofore been used as automatic transmission fluids, are generally incapable of meeting both high and low temperature performance requirements. Petroleum oils can be modified, for example, by addition of kerosene to improve low temperature properties, however, the fluids then become too volatile for continued high speed, high temperature operation. Conversely, petroleum oils can be modified to provide good high temperature properties, but such compositions generally become too viscous at low temperatures and thus do not function properly in cold weather.
Known synthetic esters while being capable of imparting some improved properties over petroleum based fluids, have generally been found unsuitable for use as automatic transmission fluids. One reason for the lack of acceptance of synthetic esters as automatic transmission fluids is due to their incompatibility with the seals used in automatic transmissions. Particularly preferred elastomers used in automatic transmissions are the copolymers of butadiene-acrylonitrile (Buna-N) and polyacrylates. The incompatibility of synthetic esters with such seals results in insufficient or excessive swelling of the seal.
Insufficient or excessive swell causes the seals to lose their ability to retain and confine the transmission fluid. Leakage occurs which can cause high fluid consumption.
Controlled seal swell is, therefore, an important characteristic of an acceptable automatic transmission fluid. It is essential that the fluid be capable of imparting a controlled swelling of the transmission seals to prevent leakage of the fluids.
By the use of the term "seal swell" it is meant the amount, in percent, that the volume of a seal expands or contracts upon contact with, and exposure to, the transmission fluid under transmission operating conditions.
Typically, seal swell is measured by ASTM standard D 471-72.
DEXRON II is a General Motors' automatic transmission fluid. It is highly desirable that an automatic transmission fluid meet all the specifications for a DEXRON II fluid in order to be commercially acceptable (Specification GM 6137-M). This entire GM Specification is incorporated herein by reference. The DEXRON II specifications require a Buna-N seal swell of from +1.% to +5.%, a polyacrylate seal swell of 0 to +10% and a silicone seal swell of 0 to +5%. The hardness change limits are 0 to -5, 0 to +5 and 0 to -10, respectively.
It is also highly desirable that an automatic transmission fluid meet all of the Ford M-2C33G specifications. This entire Ford Specification is incorporated herein by reference.
An automatic transmission fluid must also have an acceptable viscosity range to enable it to be liquid at temperatures as low as minus 40.degree. F. and still retain film strength to adequately lubricate at operating temperatures that can approach 350.degree. F. The viscosity characteristics of an automatic transmission fluid must be such that the fluid can be used over a wide temperature range, that is, adequate viscosity at high temperature, comparatively low viscosity at low temperatures and a low rate of change of viscosity with temperature.
Typically, viscosities are measured by ASTM standard D445 at 210.degree. F., 100.degree. F., 0.degree. F. and -20.degree. F. A satisfactory automatic transmission fluid should have a viscosity within the DEXRON II specifications:
______________________________________ Viscosity (centistokes, cS) at 210.degree. F.: 5.5 minimum Brookfield Viscosity (centispoise, cP) at -10.degree. F.: 4,000 maximum at -40.degree. F.: 50,000 maximum. ______________________________________
It is also highly desirable that an automatic transmission fluid have a viscosity within the Ford M-2C33G specifications:
______________________________________ Viscosity (centistokes, cS) at 210.degree. F. 6.35 minimum Brookfield Viscosity (centipoise, cP) at 0.degree. F.: 1,400 maximum at -40.degree.F.: 40,000 maximum. ______________________________________
The resistance of a fluid to viscosity change with a change in temperature is known as the viscosity index. In general, the greater the viscosity index the more desirable the fluid because of the greater resistance to thickening at low temperatures and to thinning out at high temperatures. A number of additives can be used to improve the viscosity index of a base ester fluid. Under high shear and under high temperatures and oxidation conditions, viscosity index improvers tend to break down and lose their ability to thicken the fluid. This causes the fluid to revert to the viscosity index of the base ester fluid. When the viscosity index decreases to an unsatisfactory level, the fluid must be replaced. Thus in order to insure that replacement of the transmission fluid will not occur too frequently, it is desirable that the transmission fluid have a satisfactory viscosity index without the need for a viscosity index improver, or at the very least, with a minimum amount of viscosity index improver, i.e. no greater than about 10%, and preferably no greater than about 3.%.
It is therefore desirable to have an automatic transmission fluid which has a viscosity index of at least about 125, without a viscosity index improver.
Typically, viscosity index is measured by ASTM Standard D 2270.
Coupled with an acceptable viscosity and viscosity index, a satisfactory automatic transmission fluid should have an acceptable pour point. The term "pour point" means the lowest test temperature at which the fluid will flow. Typically, pour point is measured by ASTM Standard D-97. A satisfactory automatic transmission fluid should have a pour point no higher than about -40.degree. F.
Additionally, a satisfactory automatic transmission fluid must have suitable oxidation stability at least as good as a DEXRON II fluid or the Ford M-2C33G specifications. Oxidation stability can be measured by Federal Test Method Standard 791B, Method No. 5307, at various temperatures.
It is generally recognized in the art that a base fluid alone may not be able to meet all the foregoing standards. To meet these standards, the base fluid must be fortified with one or more additives to improve its characteristics. Thus, an automatic transmission fluid generally has contained therein, in addition to the base fluid, certain additives to optimize certain characteristics of the fluid. Thus, the base fluid used in automatic transmission fluids must also have "additive compatibility" to benefit from the incorporation of these additives. Many base fluids, including certain polyol esters, do not exhibit such additive compatibility. The term "additive compatibility" means the ability of the base fluid to dissolve additives without hazing, flocculating or settling out.
A non-limiting list of the more common type additives are:
(a) Oxidation Inhibitors; PA1 (b) Rust Inhibitors; PA1 (c) Copper Corrosion Inhibitors; PA1 (d) Antiwear Agents; PA1 (e) Detergents and Dispersants; PA1 (f) Viscosity-Index Improvers; PA1 (g) Foam Inhibitors; and PA1 (h) Low Viscosity Diluents. PA1 (i) a low molecular weight acid composition containing at least one saturated monocarboxylic acid of from 3 to 8 carbon atoms, and PA1 (ii) an alpha-branched acid composition containing at least one saturated monocarboxylic acid of from 11 to 13 carbon atoms of which at least about 40% by weight is alpha-branched monocarboxylic acid; PA1 (a) U.S. Pat. No. 3,579,551 to Craddock et al; PA1 (b) U.S. Pat. No. 3,579,552 to Craddock et al; PA1 (c) U.S. Pat. No. 3,816,488 to Craddock et al; PA1 (d) U.S. Pat. No. 3,816,489 to Craddock et al; PA1 (e) U.S. Pat. No. 3,816,490 to Forster et al; PA1 (f) U.S. Pat. No. 3,818,060 to Forster et al; PA1 (g) U.S. Pat. No. 3,821,265 to Forster et al; PA1 (h) U.S. Pat. No. 4,000,170 to Forster et al; PA1 1,5-hexane diol; PA1 (a) Oxidation Inhibitors; PA1 (b) Rust Inhibitors; PA1 (c) Copper Corrosion Inhibitors; PA1 (d) Antiwear Agents; PA1 (e) Detergents and Dispersants; PA1 (f) Viscosity Index Improvers; PA1 (g) Foam Inhibitors; and PA1 (h) Low Viscosity Diluents.
Several types of additives are generally added to a base fluid to form an automatic transmission fluid. A particular additive composition may serve a plurality of functions in the fluid.
The present invention has for an object a functional fluid containing a base ester fluid either alone or in combination with additives, which is capable of meeting the standards for a satisfactory automatic transmission fluid.
It is still another object of this invention to provide a base ester fluid for use in the automatic transmission fluid of this invention.